1. Field of Invention
This invention relates to induced current linear and rotary position transducers. In particular, this invention is directed to an induced current position transducer with an improved multi-tap winding configuration to increase signal sensitivity and provide a digital interpolation capability.
2. Description of Related Art
U.S. patent application Ser. No. 08/441,769, filed May 16, 1995, and incorporated herein in its entirety, discloses an incremental induced current position transducer. U.S. patent application Ser. No. 08/645,483, filed May 13, 1996, and incorporated herein in its entirety, discloses an electronic caliper using an induced current position transducer. Both applications disclose associated signal processing techniques for induced current position transducers.
The operation of the induced current position transducers of these applications is generally shown in FIGS. 1 and 2. As shown in FIG. 1, the induced current position transducer 100 comprises a read head 120 that is movable relative to a scale 110. The scale 110 is preferably formed on a printed circuit board using standard printed circuit board technology. The read head 120 can also be formed on a printed circuit board. However, the read head 120 can also be formed on an integrated circuit (IC), preferably a silicon IC using standard silicon IC processing techniques.
A plurality of magnetic flux modulators 112 are distributed along a measuring axis 114 of the induced current position transducer 100 at a pitch equal to a wavelength .lambda., which is described in more detail below. The flux modulators 112 have a nominal width along the measuring axis 114 of .lambda./2. The flux modulators 112 have a width d in a direction perpendicular to the measuring axis 114.
The read head 120 includes a generally rectangular transmitter winding 122 that is connected to a drive signal generator 150. The drive signal generator 150 provides a time-varying drive signal to the transmitter winding 122. The time-varying drive signal is preferably a high frequency sinusoidal signal, a pulse signal, or an exponentially decaying sinusoidal signal. When the time-varying drive signal is applied to the transmitter winding 122, the time-varying current flowing in the transmitter winding 122 generates a corresponding time-varying, or changing, magnetic field. The transmitter winding 122 is generally rectangularly shaped, and the dimensions are chosen such that the generated magnetic field is substantially constant within a flux region in the central portion of the transmitter winding 122.
The read head 120 further includes a first receiver winding 124 and a second receiver winding 126 positioned on the read head within the flux region inside the transmitter winding 122. Each of the first receiver winding 124 and the second receiver winding 126 is formed by a plurality of first loop segments 128 and second loop segments 129. The first loop segments 128 are formed on a first surface of a layer of the printed circuit board or IC. The second loop segments 129 are formed on another surface of the layer of the printed circuit board or IC. The layer of the printed circuit board or IC acts as an electrical insulation layer between the first loop segments 128 and the second loop segments 129. Each end of each of the first loop segments 128 is connected to one end of one of the second loop segments 129 through vias 130 formed in the layer of the printed circuit board or IC.
The first and second loop segments 128 and 129 are suitably sinusoidally shaped. Accordingly, as shown in FIG. 1, the first and second loop segments 128 and 129 forming each of the receiver windings 124 and 126 form a sinusoidally-shaped periodic pattern having the wavelength .lambda.. Each of the receiver windings 124 and 126 are thus formed having a plurality of loops 132 and 134.
The loops 132 and 134 in each of the first and second receiver windings 124 and 126 has a width along the measuring axis 114 equal to .lambda./2. Thus, each pair of adjacent loops 132 and 134 has a width equal to .lambda.. Furthermore, the first and second loop segments 128 and 129 go through a full sinusoidal cycle in each pair of adjacent loops 132 and 134. Thus, .lambda. corresponds to the sinusoidal wavelength of the first and second receiver windings 124 and 126. Furthermore, the receiver winding 126 is offset by .lambda./4 from the first receiver winding 124 along the measuring axis 114. That is, the first and second receiver windings 124 and 126 are in quadrature.
The changing drive signal from the drive signal generator 150 is applied to the transmitter winding 122 such that current flows in the transmitter winding 122 from a first terminal 122a, through the transmitter winding 122 and out through a second terminal 122b. Thus, the magnetic field generated by the transmitter winding 122 descends into the plane of FIG. 1 within the transmitter winding 122 and rises up out of the plane of FIG. 1 outside the transmitter winding 122. Accordingly, the changing magnetic field within the transmitter winding 122 generates an induced electromotive force (EMF) in each of the loops 132 and 134 formed in the receiver windings 124 and 126.
The loops 132 and 134 have opposite winding directions. Thus, the EMF induced in the loops 132 has a polarity that is opposite to the polarity of the EMF induced in the loops 134. The loops 132 and 134 enclose the same size areas and thus nominally the same amount of magnetic flux. Therefore, the absolute magnitude of the EMF generated in each of the loops 132 and 134 is nominally the same.
There are preferably equal numbers of loops 132 and 134 in each of the first and second receiver windings 124 and 126. Ideally, the positive polarity EMF induced in the loops 132 is exactly offset by the negative polarity EMF induced in the loops 134. Accordingly, the net nominal EMF on each of the first and second receiver windings 124 and 126 is zero and it is intended that no signal is output from the first and second receiver windings 124 and 126 as a result solely of the direct coupling from the transmitter winding 122 to the receiver windings 124 and 126.
When the read head 120 is placed in proximity to the scale 110, the changing magnetic flux generated by the transmitter winding 122 also passes through the flux modulators 112. The flux modulators 112 modulate the changing magnetic flux and can be either flux enhancers or flux disrupters.
When the flux modulators 112 are provided as flux disrupters, the flux modulators 112 are formed as raised portions of a conductive substrate, i.e., like gear teeth, or preferably as conductive plates, or thin conductive films on the scale 110. As the changing magnetic flux passes through the conductive plates, raised conductive portions, or thin films, eddy currents are generated in the conductive plates, raised conductive portions, or thin films. These eddy currents in turn generate magnetic fields having a field direction that is opposite to that of the magnetic field geneated by the transmitter winding 122. Thus, in areas proximate to each of the flux disrupter-type flux modulators 112, the net magnetic flux is less than the net magnetic flux in areas distant from the flux disrupter-type flux modulators 112.
When the scale 110 is positioned relative to the read head 120 such that the flux disrupters 112 are aligned with the positive polarity loops 132 of the receiver winding 124, the net EMF generated in the positive polarity loops 132 is reduced compared to the net EMF generated in the negative polarity loops 134. Thus, the receiver winding 124 becomes unbalanced and has a net negative signal across its output terminals 124a and 124b.
Similarly, when the flux disrupters 112 are aligned with the negative polarity loops 134, the net magnetic flux through the negative polarity loops 134 is disrupted or reduced. Thus, the net EMF generated in the negative polarity loops 134 is reduced relative to the net EMF generated in the positive polarity loops 132. Thus, the first receiver winding 124 has a net positive signal across its output terminals 124a and 124b.
When the flux modulators 112 are provided as flux enhancers, this result is exactly reversed. The flux-enhancer-type flux modulators 112 are formed by portions of high magnetic permeability material provided in or on the scale 110. The magnetic flux generated by the transmitter winding 122 preferentially passes through the high magnetic permeability flux-enhancer-type flux modulators 112. That is, the density of the magnetic flux within the flux enhancers 112 is enhanced, while the flux density in areas outside the flux enhancers 112 is reduced.
Thus, when the flux enhancers 112 are aligned with the positive polarity loops 132 of the second receiver winding 126, the flux density through the positive polarity loops 132 is greater than the flux density passing through the negative polarity loops 134. Thus, the net EMF generated in the positive polarity loops 132 increases, while the net EMF induced in the negative polarity loops 134 decreases. This appears as a positive signal across the terminals 126a and 126b of the second receiver winding 126.
When the flux enhancers are aligned with the negative polarity loops 134, the negative polarity loops 134 generate an enhanced EMF relative to the EMF induced in the positive polarity loops 132. Thus, a negative signal appears across the terminals 126a and 126b of the second receiver winding 126. It should also be appreciated that, as outlined in the incorporated references, both the flux enhancing and flux disrupting effects can be combined in a single scale, where the flux enhancers and the flux disrupters are interleaved along the length of the scale 110. This would act to enhance the modulation of the induced EMF, because the effects of both types of flux modulator additively combine.
As indicated above, the width and height of the flux modulators 112 are nominally .lambda./2 and d, respectively, while the pitch of the flux modulators 122 is nominally .lambda.. Similarly, the wavelength of the periodic pattern formed in the first and second receiver windings 122 and 124 is nominally .lambda. and the height of the loops 132 and 134 is nominally d. Furthermore, each of the loops 132 and 134 encloses a nominally constant area.
FIG. 2A shows the position-dependent output from the positive polarity loops 132 as the flux modulators 112 move relative to the positive polarity loops 132. Assuming the flux modulators 112 are flux disrupters, the minimum signal amplitude corresponds to those positions where the flux disrupters 112 exactly align with the positive polarity loops 132, while the maximum amplitude positions correspond to the flux disrupters 112 being aligned with the negative polarity loops 134.
FIG. 2B shows the signal output from each of the negative polarity loops 134. As with the signal shown in FIG. 2A, the minimum signal amplitude corresponds to those positions where the flux disrupters 112 exactly align with the positive polarity loops 132, while the maximum signal output corresponds to those positions where the flux disrupters exactly align with the negative polarity loops 134. It should be appreciated that if flux enhancers were used in place of flux disrupters, the minimum signal amplitudes in FIGS. 2A and 2B would correspond to the flux enhancers 112 aligning with the negative polarity loops 134, while the maximum signal amplitude would correspond to the flux enhancers 112 aligning with the positive polarity loops 132.
FIG. 2C shows the net signal output from either of the first and second receiver windings 124 and 126. This net signal is equal to the sum of the signals output from the positive and negative polarity loops 132 and 134, i.e., the sum of the signals shown in FIGS. 2A and 2B. The net signal shown in FIG. 2C should ideally be symmetrical around zero, that is, the positive and negative polarity loops 132 and 134 should be exactly balanced to produce a symmetrical output with zero offset.
However, a "DC" (position independent) component often appears in the net signal in a real device. This DC component is the offset signal V.sub.o. This offset V.sub.o, complicates signal processing. This offset has two major sources.
First, the full amplitude of the transmitter field passes through the first and second receiver windings 124 and 126. As outlined above, this induces a voltage in each loop 132 and 134. The induced voltage is nominally canceled because the loops 132 and 134 have opposite winding directions. However, to perfectly cancel the induced voltage in the receiver windings requires the positive and negative loops 132 and 134 to be perfectly positioned and shaped for a perfectly balanced result. The tolerances on the balance are critical because the voltages induced directly into the receiver winding loops 132 and 134 by the transmitter winding 122 are much stronger than the modulation in the induced voltage caused by the flux modulators 112. In practice, fabrication tolerances always prevent perfect balance.
Second, the spatially modulated field created by the flux modulators 112 also exhibits an average position-independent offset component. That is, the flux modulators 112 within the magnetic field generated by the transmitter winding 122 all create the same polarity spatial modulation in the magnetic field. For example, when flux disrupters are used, the induced eddy current field from the flux modulators has an offset because the flux disrupters within the transmitter field all create a same polarity secondary magnetic field. At the same time, the space between the flux disrupters does not create a secondary magnitude field.
Thus, each positive polarity loop 132 and each negative polarity loop 134 of the receiver windings 124 and 126 sees a net magnetic field that varies between a minimum value and a maximum value having the same polarity. The mean value of this function is not balanced around zero, i.e., it has a large nominal offset. Similarly, when flux enhancers are used, the field modulation due to the flux enhancers has a bias because the enhancers within the transmitter winding 122 all create the same field modulation, while the space between the modulators provides no modulation. Each positive and negative polarity loop 132 or 134 of each receiver winding 124 or 126 therefore sees a spatially modulated field that varies between a minimum value and a maximum value having the same polarity. The mean value of this function also has a large nominal offset.
A receiver winding having an equal number of similar positive and negative polarity loops 132 and 134 helps eliminate the offset components. However, any imperfection in the balance between the positive and negative polarity loops 132 and 134 allows residual offsets according to the previous description.
Both of these offset components are expected to be canceled solely by the symmetry between the positive and negative polarity loops 132 and 134 in the first and second receiver windings 124 and 126. This puts very stringent requirements on the manufacturing precision of the receiver windings 124 and 126. Experience in manufacturing the transducer 100 indicates it is practically impossible to eliminate this source of error from the induced current position transducer 100.
Any signal component which is independent of the transducer position, such as the aforementioned offset components, is regarded as an extraneous signal to the operation of the transducer. Such extraneous signals complicate the required signal processing circuitry.
U.S. patent application Ser. No. 08/834,432 filed Apr. 16, 1997, and incorporated herein in its entirety, discloses a reduced offset induced current position transducer. The reduced offset induced current position transducer utilizes improved winding configurations that increase the proportion of the useful output signal component relative to extraneous ("offset") components of the output signal, without requiring increased transducer fabrication accuracy. The winding configurations also provide means to enhance the degree of output signal change per unit of displacement, for a given measuring range.
The operation of the reduced offset induced current position transducer is generally shown in FIGS. 3 and 4. The transducer of FIGS. 3 and 4 produces an output type usually referred to as "incremental". "Incremental" means the transducer produces a cyclic output which is repeated according to a design-related increment of transducer displacement. As shown in FIG. 3, the reduced-offset scale 210 includes a first plurality of closed-loop coupling loops 212 interleaved with a second plurality of closed-loop coupling loops 216. Each of the coupling loops 212 and 216 is electrically isolated from the others of the first and second plurality of coupling loops 212 and 216.
Each of the first plurality of coupling loops 212 includes a first loop portion 213 and a second loop portion 214 connected by a pair of connecting conductors 215. Similarly, each of the second plurality of coupling loops 216 includes a first loop portion 217 and a second loop portion 218 connected by a pair of connecting conductors 219.
In the first plurality of coupling loops 212, the first loop portions 213 are arranged along one lateral edge of the scale 210 and are arrayed along the measuring axis 114. The second loop portions 214 are arranged along the center of the scale 210 and are arrayed along the measuring axis. The connecting conductors 215 extend perpendicularly to the measuring axis 114 to connect the first loop portions 213 to the second loop portions 214.
Similarly, in the second plurality of coupling loops 216, the first loop portions 217 are arranged along a second lateral edge of the scale 210 and arrayed along the measuring axis 114. The second loop portions 218 are arranged along the center of the scale 210 along the measuring axis, interleaved with the second loop portions 214 of coupling loops 212. The connecting conductors 219 extend generally perpendicularly to the measuring axis 114 to connect the first loop portions 217 to the second loop portions 218.
As shown in FIG. 4, the read head 220 of the transducer 200 includes a transmitter winding 222 having a first transmitter winding portion 223A and a second transmitter winding portion 223B. The first transmitter winding portion 223A is provided at a first lateral edge of the read head 220, while the second transmitter winding portion 223B is provided at the other lateral edge of the read head 220. Each of the first and second transmitter winding portions 223A and 223B have substantially the same long dimension extending along the measuring axis 114. Furthermore, each of the first and second transmitter winding portions 223A and 223B have a short dimension that extends in a direction perpendicular to the measuring axis 114 a distance d.sub.1.
The terminals 222A and 222B of the transmitter winding 222 are connected to the transmitter drive signal generator 150. The transmitter drive signal generator 150 outputs a time-varying drive signal to the transmitter winding terminal 222A. Thus, a time-varying current flows through the transmitter winding 222 from the transmitter winding terminal 222A to the transmitter terminal 222B, as indicated in FIG. 4.
In response, the first transmitter winding portion 223A generates a primary magnetic field that rises up out of the plane of FIG. 4 inside the first transmitter winding portion 223A and descends into the plane of FIG. 4 outside the loop formed by the first transmitter winding portion 223A. In contrast, the second transmitter winding portion 223B generates a primary magnetic field that rises out of the plane of FIG. 4 outside the loop formed by the second transmitter winding portion 223B and descends into the plane of FIG. 4 inside the loop formed by the second transmitter winding portion 223B. In response, a current is induced in the coupling loops 212 and 216 that counteracts the change in magnetic field.
Thus, the induced current in each of the coupling loop sections 213 and 217 flows in a direction opposite to the current flowing in the respective adjacent portions of the transmitter loops 223A and 223B. As shown in FIG. 4, adjacent ones of the second loop portions 214 and 218 in the center section of the scale have loop currents having opposite polarities. Thus, a secondary magnetic field is created having field portions of opposite polarity periodically distributed along the center section of the scale. The wavelength .lambda. of the periodic secondary magnetic field is equal to the distance between successive second loop portions 214 (or 218).
The read head 220 also includes first and second receiver windings 224 and 226 that are generally identical to the first and second receiver windings 124 and 126 shown in FIG. 1. In particular, similarly to the first and second receiver windings 124 and 126 shown in FIG. 1, the first and second receiver windings 224 and 226 are each formed by a plurality of sinusoidally shaped loop segments 228 and 229 formed on opposite sides of an insulating layer of the printed circuit board or IC forming the read head 220.
The loop segments 228 and 229 are linked through vias 230 to form alternating positive polarity loops 232 and negative polarity loops 234 in each of the first and second receiver windings 222 and 226. The receiver windings 224 and 226 are positioned in the center of the read head 220 between the first and second transmitter portions 223A and 223B. Each of the first and second receiver windings 224 and 226 extends in the direction perpendicular to the measuring axis a distance d.sub.2.
Extraneous (position independent and scale independent) coupling from the transmitter loops to the receiver loops is generally avoided in this configuration. That is, the primary magnetic fields generated by the first and second transmitter portions 223A and 223B point in opposite directions in the vicinity of the first and second receiver windings 224 and 226. Thus, the primary magnetic fields counteract each other in the area occupied by the first and second receiver windings 224 and 226. Ideally, the primary magnetic fields completely counteract each other in this area.
The first and second receiver windings 224 and 226 are spaced equal distances d.sub.3 from the inner portions of the first and second transmitter winding portions 223A and 223B. Thus, the magnetic fields generated by each of the first and second transmitter winding portions 223A and 223B in the portion of the read head 220 occupied by the first and second receiver windings 224 and 226 are in symmetric opposition. The associated inductive effects thus effectively cancel each other out. Thus, the net voltage induced in the first and second receiver windings 224 and 226 by extraneous direct coupling to the first and second transmitter winding portions 223A and 223B is reduced to a first extent by positioning the transmitter windings away from the receiver windings. Secondly, the symmetric design effectively reduces the net extraneous coupling to zero.
Each of the first plurality of coupling loops 212 is arranged at a pitch equal to a wavelength .lambda. of the first and second receiver windings 224 and 226. Furthermore, the first loop portions 213 each extends a distance along the measuring axis 114 which is as close as possible to the wavelength .lambda. while still providing the insulating space 201 between adjacent ones of the first loop portions 213. In addition, the first loop portions 213 extend the distance d.sub.1 in the direction perpendicular to the measuring axis 114.
Similarly, each of the second plurality of coupling loops 216 is also arranged at a pitch equal to the wavelength .lambda.. The first loop portions 217 also extend as close as possible to each other along the measuring axis to the wavelength .lambda. while providing the space 201 between adjacent ones of the first loop portions 217. The first loop portions 217 also extend the distance d.sub.1 in the direction perpendicular to the measuring axis 114.
The second loop portions 214 and 218 of the first and second pluralities of coupling loops 212 and 216 are also arranged at a pitch equal to the wavelength .lambda.. However, each of the second loop portions 214 and 218 extends along the measuring axis as close as possible to only one-half the wavelength .lambda.. An insulating space 202 is provided between each adjacent pair of second loop portions 214 and 218 of the first and second pluralities of coupling loops 212 and 216, as shown in FIG. 4. Thus, the second loop portions 214 and 218 of the first and second pluralities of coupling loops 212 and 216 are interleaved along the length of the scale 210. Finally, each of the second loop portions 214 and 218 extends the distance d.sub.2 in the direction perpendicular to the measuring axis 114.
The second loop portions 214 and 218 are spaced the distance d.sub.3 from the corresponding first loop portions 213 and 217. Accordingly, when the read head 220 is placed in proximity to the scale 210, as shown in FIG. 4, the first transmitter winding portion 223A aligns with the first loop portions 213 of the first plurality of coupling loops 212. Similarly, the second transmitter winding portion 223B aligns with the first loop portions 217 of the second plurality of coupling loops 216. Finally, the first and second receiver windings 224 and 226 align with the second loop portions 214 and 218 of the first and second coupling loops 212 and 216.
In operation, a time-varying drive signal is output by the transmitter drive signal generator 150 to the transmitter winding terminal 222A. Thus, the first transmitter winding portion 223A generates a first changing magnetic field having a first direction while the second transmitter winding portion 223B generates a second magnetic field in a second direction that is opposite to the first direction. This second magnetic field has a field strength that is equal to a field strength of the first magnetic field generated by the first transmitter winding portion 223A.
Each of the first plurality of coupling loops 212 is inductively coupled to the first transmitter winding portion 223A by the first magnetic field generated by the first transmitter winding portion 223A. Thus, an induced current flows clockwise through each of the first plurality of coupling loops 212. At the same time, the second plurality of coupling loops 216 is inductively coupled to the second transmitter winding portion 223B by the second magnetic field generated by the second transmitter winding portion 223B. This induces a counterclockwise current to flow in each of the second plurality of coupling loops 216. That is, the currents through the second portions 214 and 218 of the coupling loops 212 and 216 flow in opposite directions.
The clockwise flowing current in each of the second portions 214 of the first coupling loops 212 generates a third magnetic field that descends into the plane of FIG. 4 within the second portions 214. In contrast, the counterclockwise flowing currents in the second loop portions 218 of the second coupling loops 216 generate a fourth magnetic field that rises out of the plane of FIG. 4 within the second loop portions 218 of the second coupling loops 216. Thus, a net alternating magnetic field is formed along the measuring axis 114. This net alternating magnetic field has a wavelength which is equal to the wavelength .lambda. of the first and second receiver windings 224 and 226.
Accordingly, when the positive polarity loops 232 of the first receiver winding 224 are aligned with either the second loop portions 214 or 218, the negative polarity loops 234 of the first receiver winding 224 are aligned with the other of the second loop portions 214 or 218. This is also true when the positive polarity loops 232 and the negative polarity loops 234 of the second receiver winding 226 are aligned with the second loop portions 214 and 218. Because the alternating magnetic field generated by the second loop portions 214 and 218 is spatially modulated at the same wavelength as the spatial modulation of the first and second receiver windings 214 and 216, the EMF generated in each of the positive and negative polarity loops 232 and 234 when aligned with the second loop portions 214 is equal and opposite to the EMF generated when they are aligned with the second loop portions 218.
Thus, the net output of the positive polarity loops 232, as the read head 220 moves relative to the scale 210 is a sinusoidal function of the position "x" of the read-head along the scale and the offset component of the output signal due to extraneous coupling is nominally zero. Similarly, the net output from the negative polarity loops 234, as the read head 220 moves relative to the scale 210, is also a sinusoidal function of the position "x" of the read head along the scale and the offset component of the output signal due to extraneous coupling is nominally zero. The EMF contributions from the positive polarity loops 232 and the negative polarity loops 234 are in phase. They thus generate a net position-dependent output signal corresponding to FIG. 2C, but the DC bias V.sub.o due to extraneous coupling is reduced to insignificance in this preferred embodiment.
Finally, the first and second receiver windings 224 and 226, like the first and second receiver windings 124 and 126, are in quadrature. Thus, the output signal generated by the first receiver winding 224 as a function of x and output to the receiver signal processing circuit 140 is 90.degree. out of phase with the signal output by the second receiver winding 226 as a function of x to the receiver signal processing circuit 140.
The receiver signal processing circuit 140 inputs and samples the output signals from the first and second receiver windings 224 and 226, converts these signals to digital values and outputs them to the control unit 160. The control unit 160 processes these digitized output signals to determine the relative position x between the read head 220 and the scale 210 within a wavelength .lambda..
Based on the nature of the quadrature output from the first and second receiver windings 224 and 226, the control unit 160 is able to determine the direction of relative motion between the read head 220 and the scale 210. The control unit 160 counts the number of partial or full "incremental" wavelengths .lambda. traversed, by signal processing methods well-known to those skilled in the art and disclosed in the incorporated references. The control unit 160 uses that number and the relative position within a wavelength .lambda. to output the relative position between the read head 220 and the scale 210 from a set origin. The control unit 160 also outputs control signals to the transmitter drive signal generator 150 to generate the time-varying transmitter drive signal.
All of the above-described inductive transducers would benefit from improved signal sensitivity and more accurate interpolation of position increments smaller than a wavelength of the modulation of the magnetic field in the transducer.